The present invention pertains to forming a film bulk acoustic resonator (xe2x80x9cFBARxe2x80x9d) structure. More specifically, the present invention relates to the methods of forming a structure for a film bulk acoustic resonator having high-Q and low insertion loss.
In some instances it is desirable to provide a radio frequency front-end filter. In the past ceramic filters and SAW filters have been used as front-end radio frequency filters. There are problems with SAW filters in that such filters start to have excessive insertion loss above 2.4 gigahertz (GHz). Ceramic filters are large in size and can only be fabricated with increasing difficulty as the frequency increases.
A basic FBAR device 100 is schematically shown in FIG. 1. The FBAR device 100 is formed on the horizontal plane of a substrate 110. A first layer of metal 120 is placed on the substrate 110, and then a piezoelectric layer 130 is placed onto the metal layer 120. The piezoelectric layer can be ZnO, AIN, PZT, any other piezoelectric materials. A second layer of metal 122 is placed over the piezoelectric layer 130. The first metal layer 120 serves as a first electrode 120 and the second metal layer 122 serves as a second electrode 122. The first electrode 120, the piezoelectric layer 130, and the second electrode 122 form a stack 140. A portion of the substrate 110 behind or beneath the stack 140 is removed using back side bulk silicon etching. The back side bulk silicon etching is done using deep trench reactive ion etching or using a crystallographic-orientation-dependent etch, such as KOH, TMAH, and EDP. Back side bulk silicon etching produces an opening 150 in the substrate 110. The resulting structure is a horizontally positioned piezoelectric layer 130 sandwiched between the first electrode 120 and the second electrode 122 positioned above the opening 150 in the substrate. The FBAR is a membrane device suspended over an opening in a horizontal substrate.
FIG. 2 illustrates the schematic of an electrical circuit 200 which includes a film bulk acoustic resonator 100. The electrical circuit 200 includes a source of radio frequency xe2x80x9cRFxe2x80x9d voltage 210. The source of RF voltage 210 is attached to the first electrode 120 via electrical path 220 and attached to the second electrode 122 by the second electrical conductor 222. The entire stack 140 can freely resonate in the Z direction xe2x80x9cd33xe2x80x9d mode when the RF voltage at resonant frequency is applied. The resonant frequency is determined by the thickness of the membrane or the thickness of the piezoelectric layer 130 which is designated by the letter xe2x80x9cdxe2x80x9d or dimension xe2x80x9cdxe2x80x9d in FIG. 2. The resonant frequency is determined by the following formula:
f0xcx9cV/2d, where
f0=the resonant frequency,
V=acoustic velocity of piezoelectric layer, and
d=the thickness of the piezoelectric layer.
It should be noted that the structure described in FIGS. 1 and 2 can be used either as a resonator or as a filter. To form an FBAR, piezoelectric films, such as ZnO and AlN, are used as the active materials. The material properties of these films, such as the longitudinal piezoelectric coefficient and acoustic loss coefficient, are key parameters for the resonator""s performance. Key performance factors include Q-factors, insertion loss, and the electrical/mechanical coupling. Currently, to manufacture an FBAR the piezoelectric film is deposited on a metal electrode using reactive sputtering. The resulting films are polycrystalline with a c-axis texture orientation. In other words, the c-axis is perpendicular to the substrate. This processing procedure has several problems.
An FBAR is formed as a piezoelectric layer sandwiched between two electrodes. Top and bottom electrodes are necessary for electrical outpur of the FBAR Therefore a bottom electrode is required. The starting layer or seed layer for the piezoelectric film deposition for FBAR has been limited to conductive materials. Any other non-conductive or single-crystal materials, which could induce very high-quality or single-crystal piezoelectric films, can not be used as the seed layer using conventional FBAR fabrication techniques.
When a piezoelectric film is sputtered onto a conductive metal, the initial layer of approximately 0.05 um of the sputtered film typically consists of a polycrystalline material with partially developed texture. This initial layer has poor piezoelectric effect. This degrades the overall film quality. This becomes a performance issue for high frequency FBARs having a resonance frequency of 10 GHz or above which has a piezoelectric film about 0.2 um thick.
Thus, there is need for an FBAR device and a method for producing an FBAR device that results in a single-crystal piezoelectric film. There is also a need for a method of fabricating an FBAR device that has good performance qualities and which uses a seed layer other than a highly conductive electrode. There is also a need for a fabrication technique where an initial sputtered layer of piezoelectric material can be removed since this layer may be polycrystalline and have poor piezoelectric effect.